The present invention relates to a neutron monitoring system for detecting neutron fluxes using a neutron detector that outputs pulse signals upon detecting the neutron flux.
Heretofore in a nuclear power plant, the nuclear reactor output is monitored by counting the number of pulse signals being output from a neutron detector per unit time. The neutron detector generates negative pulse signals.
The pulse signals generated from the neutron detector is normally converted and amplified by a pre-amplifier into voltage pulse signals, and pulse height discrimination is performed thereto. When the negative pulse signal is smaller (greater in absolute value) than the preset value, the pulse height discrimination determines that the signal is a neutron flux detection pulse signal, and thus the influence of noises is removed.
However, welding work is performed during periodic inspection in the nuclear power plant, generating electric noise signals (surge noises) greater than the pulse height discrimination level (preset value). Therefore, the electric noise signals are counted as the detection pulse signals of the neutron flux, causing pulse rate counting error. The electric noise signals are generated both in positive and negative polarities.
In order to prevent the pulse rate counting error caused by the surge noises when measuring the neutron flux, pulse height discrimination of the noise signal in the positive polarity is performed so as to detect the noise signal, and when a noise signal is detected, the count rate of the neutron detection pulse signals is corrected. According to the count rate correction, when a noise signal is detected, the counting of the neutron detection pulse signals is stopped for a predetermined time. Such system is disclosed for example in Japanese Patent Laid-Open Publication No. 8-82681.
According to the prior art system, when a noise signal is detected during measurement of pulse counts per unit time, the counting of the neutron detection pulse signals is stopped for a predetermined time. The unit time of measurement cycle in a digital neutron measurement device is approximately 200 ms. The count of the neutron detection pulse signals ranges between 10 and 106.
Accordingly, if the counting of signals is stopped for a predetermined time when there is a great number of neutron detection pulse signals being output, the measurement accuracy is deteriorated greatly, and when plural noise signals are detected within a unit time, it becomes impossible to count the signals.
The present invention aims at solving the problems of the prior art mentioned above, and the object of the invention is to provide a neutron monitoring system capable of measuring the neutron fluxes reliably with high accuracy.
The present invention characterizes in counting the negative pulse signals output from the neutron detector and also counting the positive noise pulse signals, and subtracts the positive pulse count from the negative pulse count per unit time in order to measure the neutrons.
In other words, the present invention determines the negative pulse count by subtracting the positive pulse count or noise pulses from a negative pulse count which is the sum of noise pulses and neutron flux detection pulses counted within a measurement cycle per unit time, and sets the negative pulse count as the measured neutron value.
The present invention measures the neutrons based only on the negative pulse signals corresponding to the pulse signals output when the neutron fluxes are detected, by canceling the noise pulses detected in both positive and negative polarities. Therefore, the invention enables to measure the neutrons with high accuracy without being influenced by the number of noise pulses detected per unit time.